linux-kernel - Re: [PATCH 11/16] gpu: nova-core: add falcon register definitions and base code

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Message-Id: <D9KVS9Y14F7Y.2ZE3NDGDKD0LO@nvidia.com>
Date: Thu, 01 May 2025 23:07:25 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Joel Fernandes" <joelagnelf@...dia.com>, "Danilo Krummrich"
 <dakr@...nel.org>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "David Airlie"
 <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "Jonathan Corbet"
 <corbet@....net>, "John Hubbard" <jhubbard@...dia.com>, "Ben Skeggs"
 <bskeggs@...dia.com>, "Timur Tabi" <ttabi@...dia.com>, "Alistair Popple"
 <apopple@...dia.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
 <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 11/16] gpu: nova-core: add falcon register definitions
 and base code

On Thu May 1, 2025 at 9:22 AM JST, Joel Fernandes wrote:
> Hi Alex,
>
> On 4/30/2025 8:09 PM, Alexandre Courbot wrote:
>>>> I am just not seeing the benefits of not using dyn for
>>>> this use case and only drawbacks. IMHO, we should try to not be doing the
>>>> compiler's job.
>>>>
>>>> Maybe the only benefit is you don't need an Arc or Kbox wrapper?
>>> That's not a huge concern for me, it's only one single allocation per Engine,
>>> correct?
>> Correct. Note that for other engines we will be able to store the HALs as
>> static singletons instead of building them on the heap like I am
>> currently doing. The reason for doing this on falcon is that the
>> dual-dimension of the instances makes it more complex to build and look
>> them up.
>> 
>> ... or maybe I could just use a macro? Let me try that and see whether
>> it works.
>
> Do you mean a macro for create_falcon_hal which adds an entry to this?
>
>  let hal = match chipset {
>     Chipset::GA102 | Chipset::GA103 | Chipset::GA104 | Chipset::GA106
> |Chipset::GA107 => { .. }
>
>
> Actually it would be nice if a single macro defined both a chipset and created
> the hal together in the above list, that way the definition of a "chipset" is in
> a singe place. Kind of like what I did in the vbios patch for various BiosImage.
> But not sure how easy it is to do for Falcon.
>
> Or perhaps you meant a macro that statically allocates the Engine + HAL
> combination, and avoids need for Arc/KBox and their corresponding allocations?

I was thinking of a macro to create all the Chipset * Engine static
instances of HALs, and generate the body of a lookup function to return
the right one for a given Chipset at runtime.

But trying to write it, I realized it wasn't as easy as I thought since
generics cannot be used as macro parameters - i.e. if you have 
<E: Engine> as a generic and pass `E` to the macro, it will see... `E`
and not whatever was bound to `E` when the code is monomorphized (macros
are expanded before generics it seems).

A solution to that would involve new traits and a bunch of boilerplate,
which I have decided is not worth the trouble to save one 8-bytes object
on the heap per falcon instance. :) I'll keep things as they currently
are for now.